The dynamic chamber method: trace gas exchange fluxes (NO, NO₂, O₃) between plants and the atmosphere in the laboratory and in the field

Abstract. We describe a dynamic chamber system to determine reactive trace gas exchange fluxes between plants and the atmosphere under laboratory and, with small modifications, also under field conditions. The system allows measurements of the flux density of the reactive NO-NO₂-O₃ triad and additionally of the non-reactive trace gases CO₂ and H₂O. The chambers are made of transparent and chemically inert wall material and do not disturb plant physiology. For NO₂ detection we used a highly NO₂ specific blue light converter coupled to chemiluminescence detection of the photolysis product, NO. Exchange flux densities derived from dynamic chamber measurements are based on very small concentration differences of NO₂ (NO, O₃) between inlet and outlet of the chamber. High accuracy and precision measurements are therefore required, and high instrument sensitivity (limit of detection) and the statistical significance of concentration differences are important for the determination of corresponding exchange flux densities, compensation point concentrations, and deposition velocities. The determination of NO₂ concentrations at sub-ppb levels (<1 ppb) requires a highly sensitive NO/NO₂ analyzer with a lower detection limit (3σ-definition) of 0.3 ppb or better. Deposition velocities and compensation point concentrations were determined by bi-variate weighted linear least-squares fitting regression analysis of the trace gas concentrations, measured at the inlet and outlet of the chamber. Performances of the dynamic chamber system and data analysis are demonstrated by studies of Picea abies L. (Norway Spruce) under field and laboratory conditions. Our laboratory data show that the quality selection criterion based on the use of only significant NO₂ concentration differences has a considerable impact on the resulting compensation point concentrations yielding values closer to zero. The results of field experiments demonstrate the need to consider photo-chemical reactions of NO, NO₂, and O₃ inside the chamber for the correct determination of the exchange flux densities, deposition velocities, as well as compensation point concentrations. Under our field conditions NO₂ deposition velocities would have been overestimated up to 80%, if NO₂ photolysis has not been considered. We also quantified the photolysis component for some previous NO₂ flux measurements. Neglecting photo-chemical reactions may have changed reported NO₂ compensation point concentration by 10%. However, the effect on NO₂ deposition velocity was much more intense, ranged between 50 and several hundreds percent. Our findings may have consequences for the results from previous studies and ongoing discussion of NO₂ compensation point concentrations.